I Love Leucine

Jose Antonio

The amazing muscle- and power-building effects of amino acids

†Do you remember HMB? Many years ago it was touted as the true anabolic metabolite derived from the oh-so-pedestrian amino acid leucine. In the past decade the indications have become clearer that leucine probably exerts myriad effects related to protein anabolism. For instance, during exercise, muscle protein synthesis decreases and protein degradation and stimulation of branched-chain amino acid oxidation increase (the BCAAs are leucine, valine and isoleucine). The decrease in protein synthesis is associated with inhibition of protein factors that are regulated by intracellular insulin signaling and leucine concentrations.

In essence, both insulin and leucine are key regulators of muscle protein synthesis.1 Leucine by itself increases that process.2 By combining it with protein and carbohydrate, you get quite an anabolic supereffect.

For example, in one study eight male subjects were randomly assigned to three trials in which they downed drinks containing carbohydrate; carbohydrate and protein; or carbohydrate, protein and free leucine following 45 minutes of resistance exercise. The researchers discovered that plasma insulin response was higher in the leucine group than in the other two. Whole-body protein breakdown rates were lower, and whole-body protein synthesis rates were higher in both groups that got protein. Moreover, the addition of leucine resulted in a lower protein oxidation rate. To top it off, muscle protein synthesis, measured over a six-hour period of postexercise recovery, was significantly greater in the leucine group.3

Okay, this study looked mainly at short-term effects. What happens if you supplement with leucine? A recent study looked at the effects of dietary leucine supplementation on the exercise performance of outrigger canoeists. Thirteen competitive outrigger canoeists, 10 female and three male, underwent testing before and after six-week supplementation with capsules of either L-leucine or a placebo. Testing included anthropometry, 10-second upper-body power and work and a row to exhaustion at 70 to 75 of percent maximal aerobic power. Perceived exertion (RPE), heart rate (HR) and plasma BCAA and tryptophan concentrations were assessed.

What happened? Leucine supplementation resulted in significant increases in plasma leucine and total BCAA concentrations. Upper-body power and work significantly increased in both groups after supplementation, but power was significantly greater after leucine supplementation compared to the placebo. Rowing time significantly increased and average RPE significantly decreased with leucine supplementation, while the variables were unchanged with the placebo. Leucine supplementation had no effect on the plasma tryptophan-to-BCAA ratio, HR or anthropometric variables. Six weeks of dietary leucine supplementation significantly improved endurance performance and upper-body power in the outrigger canoeists without significant change in their plasma ratio of tryptophan to BCAA.4

So there you have it. Leucine regulates muscle protein synthesis and improves muscular power and endurance.